A gas turbine engine includes a compressor for providing compressed air which is mixed with fuel in a combustor and ignited for generating combustion gases which flow through a turbine for generating power. The turbine includes one or more stages, with each stage including a plurality of circumferentially spaced rotor blades extending from a disc which is in turn joined to a shaft for providing power to the compressor, for example. Disposed upstream of each rotor blade stage is a turbine nozzle including a plurality of circumferentially spaced stator vanes for suitably channeling the combustion gases to the respective rotor blades.
The stator vanes and rotor blades are conventionally cooled using a portion of the compressed air to provide acceptable life in operation under the adverse affects of the hot combustion gases. Depending upon the designed-for combustion gas temperatures generated by the combustor, various types of cooling schemes are used for effectively cooling the vanes and blades. Such schemes include conventionally known film cooling wherein a plurality of film cooling apertures are disposed through the airfoils of the vanes and blades, and the compressed air is channeled through the airfoils and out the holes for effecting a layer of film cooling air along the outer surface of the airfoils which provides a barrier against the combustion gases flowable thereover. Since the leading edge of the airfoil is typically subject to the highest heat transfer coefficient it therefore experiences the highest heat flux into the airfoil thusly requiring a correspondingly greater amount of heat transfer therefrom for providing effective cooling thereof. And, since downstream of the airfoil leading edge the heat flux decreases, less heat transfer is required for the effective cooling thereof.
In another cooling scheme, a conventional hollow impingement baffle is disposed inside the airfoil and spaced away from the inner surface thereof, with the baffle including impingement holes sized for effecting impingement jets of cooling air against the inner surface of the airfoil for providing impingement cooling thereof. The spent impingement air is then discharged from the airfoil either through the film cooling holes therethrough, or through conventional trailing edge apertures, for example.
Again, the greatest amount of cooling or heat transfer is required in the high heat flux leading edge region as compared to low heat flux region near the airfoil mid-chord, for example. Such heat transfer may be obtained by using impingement cooling, or film cooling, or both in accordance with conventional practice.
However, with a single supply pressure of the cooling air to a hollow airfoil, it is difficult to simultaneously provide adequate cooling of the high heat flux leading edge region and uniform cooling of the low heat flux mid-chord region extending downstream therefrom with reduced total airflow.
For example, impingement cooling requires a given, relatively high pressure ratio across the impingement baffle to drive the cooling air through the impingement holes in impingement against the airfoil inner surface to match the highest heat flux region at the leading edge. Since the pressure ratio across the baffle is driven by the supply pressure on its inside relative to the discharge pressure on its outside, the single, high supply pressure required for the high heat flux region leads to a compromise for the low heat flux region.
More specifically, impingement jet cooling is a function of the hole density, or number of holes per unit area, and the driving pressure ratio thereacross which will effect a specific average metal temperature of the airfoil. Most cooling from an impingement jet is located directly below an impingement hole with least cooling occurring between adjacent holes. Impingement jet cooling therefore effects local variations in airfoil temperature in a generally sinusoidal pattern from jet-to-jet with a resulting average temperature due thereto. The variations are referred to as hot and cold spots associated with the airfoil between and below the impingement holes, respectively.
In designing effective cooling of the airfoil, the difference in temperature between the hot and cold spots should be as low as possible for obtaining a desired average temperature since the hot and cold spots can decrease the effective useful life of the airfoil. By increasing the hole density, both the average metal temperature and the difference in magnitude between the hot and cold spots may be reduced but at the expense of an increase in total cooling airflow channeled through the increased collective flow area of the higher density holes.
However, compressor air used for cooling the airfoils necessarily decreases overall efficiency of the gas turbine engine since it is being used for cooling purposes and does not undergo combustion with the attendant power generation therefrom. Accordingly, conventional cooling schemes utilize as few cooling air apertures as practical for minimizing the required amount of cooling air while still providing effective average cooling of the airfoil without unacceptably high temperature fluctuations between cooling holes.
With a given pressure ratio across the impingement baffle, and with a common supply pressure of the compressed air to the inside of the baffle, the hole density may be preselected to ensure adequate average cooling of the high heat flux region adjacent the leading edge which, however, provides overcooling of the airfoil downstream of the leading edge for a hole density selected to limit hot and cold spots. Alternatively, if the hole density is selectively decreased downstream of the leading edge to provide a lower heat transfer and less cooling thereof to prevent overcooling, the temperature variations between adjacent holes increases for a given desired average metal temperature thus increasing the difference in hot and cold spots. The overcooled high-density hole option wastes cooling air, while the low-density hole option increases thermally induced fatigue which may reduce the effective useful life of the airfoil. So a compromise is typically used to vary the hole density to reduce the overcooling at the expense of increased hot and cold spots.